Monday, March 30, 2009

Aerial Laser Tag, Continued

Since we have no direct synchronization b/w the blinking of the laser and the sampling of the light intensity, the quadrature aperture sampling method ensures that we can achieve this effect even though the frequency is slightly off. the end result is that we are able to detect when a modulated laser beam is shone on the light sensor whereas any other light source will be ignored.

THe laser 'gun' is made by using a microcontroller to modulate the laser beam, which is then turned on by a switch, (a trigger) which when pressed, sends out a modulated light beam for fifty milliseconds and after that, pauses for 200 milliseconds thus, only allowing firing of the gun approximately five times a second. The beam is modulated at 20 khz and the demodulation sampling techniques allows the intensity of modulated light to be registered 40 times per second.

The receiver consists of a microcontroller with a light sensor, two LEDs, and a plastic cup! The plastic cup is placed over the light sensor, diffusing all incoming light into the light sensor. Since the light sensor is so tiny, the diffuser is needed to ensure that light can reach the light sensor. Thus, when the laser is fired at the receiver, its light is diffused, and since the laser's frequency matches the desired frequency, it is accepted, and registered, turning on the LEDs to indicate a 'hit'.

We've tested pointing four other laser pointers individually and simultaneously at the receiver and only when the correct laser with the desired frequency is pointed at it, does it register and give feedback via LEDs. We have also tried using the system in direct sunlight when the laser is barely visible and it still registered, even at a distance.

So what does this mean? It means that the receiver is ignoring all other light than the desired frequency. Back to our story of the plane game, this is our prototype to see if the technique works for having an outdoor transmitter/receiver that cannot be disturbed by other light sources such as ambient light. The aim is to make multiple transmitters and receivers that can be worn by people to create a two way laser tag game between people on the ground and the plane firing lasers from overhead, and people returning shots to the plane from below. Schematics available soon, so you can build it yourself!